Advances in laser technology have led to applications in numerous new industrial and consumer products. The most commonly used laser beam scanners include polygonal mirror scanners, galvanometric scanners, holographic scanners, and acoustooptic beam deflectors. Modern laser beam scanners may be classified in two general categories: pre-objective scanners and post-objective scanners. Post-objective scanners are characterized by scanning along a curved track or image plane. In contrast, pre-objective scanners scan along a linear track or flat image plane. This invention relates primarily to pre-objective scanning configurations, although it may be adapted to a post-objective system.
The most commonly used laser beam scanners are employed in various applications, such as in laser printers, laser bar code readers, or the like. Modern laser printers employ conventional laser beam scanners which use a rotating mirror to direct a laser beam to a rotating drum. The laser beam inscribes several hundred dots per inch onto a photosensitive surface of the drum. The drum is then rotated and the photosensitive surface is treated with toner which adheres to the portions of the surface previously scanned. The drum is further rotated to transfer the toner image to a recording medium, such as paper.
Modern bar code readers employ a laser beam scanner having a mirror which guides the laser beam through a scanning sweep to read the bar code label on a product. The light output by the scanner is reflected from the product label and sensed by a photodiode. This optical signal is then converted to an electrical signal for computer use.
The use of a rotating polygonal mirror or a plain mirror is an essential component in modern scanners for precisely directing the laser beam towards a targeted scan track. Important parameters to be considered for an acceptable laser beam scanner include: scan rate, required accuracy of the system, available scan angle and system resolution, the ability to randomly access image points, power handling capability, and cost.
FIG. 1 illustrates a conventional laser beam scanner 10 having a rotating polygonal mirror 12 which rotates in a clockwise direction as indicated by arrow 14. Scanner 10 includes a laser source 16 for emitting a laser beam 18. A beam expander 20 is positioned between laser source 16 and polygonal mirror 12 to expand laser beam 18 to a laser beam 22 having a larger cross-sectional area. Expanded laser beam 22 is directed to a face 24 of rotating polygonal mirror 12. Laser beam 22 has marginal rays 26a and 26b which are deflected from face 24 of polygonal mirror 12 to provide a laser beam 28. Scanner 10 further includes scan lens 30 which focuses laser beam 28 onto a laser scan spot 32 along a scan line 34. In conventional laser beam scanners, laser spot 32 is approximately 85-140 microns in laser printer applications.
Laser beam 28 is directed onto scan lens 30 at different angles as polygonal mirror 12 is rotated. For example, as polygonal mirror 12 rotates in the clockwise direction, laser beam 28 sweeps from an initial position illustrated in dashed lines as laser beam 36 to a final position illustrated in dashed lines as laser beam 38. A sweep of laser beam 28 results in a uni-directional scan from point a to point b along scan line 34.
Polygonal mirror 12 rotates in a single direction which effectuates a one way scan along scan line 34. When laser spot 32 has traversed the scan plane from point a to point b, laser spot 32 "jumps" back to point a, leaving the return trip (i.e., a laser scan along scan line 34 from point b to point a) unused.
A rotating polygonal mirror 12 is the most popular scanning equipment component in numerous modern laser scanning devices. Unfortunately, this component is also one of the most limiting components in terms of function and cost. The polygonal mirror is usually mounted to a shaft connected to a motor which rotates the mirror at a desired angular velocity. As the weight of a polygonal mirror increases and/or the angular velocity increases, the rotating polygonal mirror experiences undesired wobbling due to irregularity in bearings. Such wobbling degrades scan trace quality and often results in a significant "across-scan error". Across-scan error is the deviation of the laser beam spot from the desired scan path. Across-scan error caused by polygonal mirror wobble increases with distance due to the divergence of the beam reflected from the facet of the polygonal mirror. Traditionally, to reduce the undesired effects of polygonal mirror wobble, scanner manufacturers have either employed precision motors with more rigid shafts, or have added error correction components to remove wobble-induced tracking errors. Various polygonal scanners and corrective measures are discussed in Optical Scanning by Gerald Marshall, chapter two, polygonal scanners, Randy J. Sherman, pages 63-123. However, incorporating a precision motor or error correction components significantly increases the fabrication cost of a laser scanning system.
Another disadvantage with using a rotating polygonal mirror is that the scanning angle is restrictive. Scan angle is directly related to the facet width of the polygonal mirror. Laser beam scanners employing polygonal mirror experience a tradeoff between scan speed and scan angle. Polygonal mirrors having a few wide facets can produce a reasonable scan angle, but only at a low scan rate. On the other hand, polygonal mirrors with a large number of facets can scan at a much higher rate, but only over a small scan angle. Alternatively, polygonal mirrors may be constructed much larger to accommodate many "wide" facets, but the mass of such mirrors are impractical in terms of manufacturing costs and the construction of more powerful motor drive systems necessary to rotate large mirrors.
My U.S. Pat. No. 5,074,628, entitled "Laser Beam Scanning Device and Method for Operation Thereof" describes a laser scanning device which reduces across-scan error. The laser scanning device employs a prism to continuously deflect a laser beam to generate a conical-shaped laser beam, which when intercepted produces an inscribed circle. The inscribed circle is collapsed by a plano-cylindrical lens and scan lens to provide uni-directional or multi-directional scanning along several different scan tracks. The plano-cylindrical lens used to collapse the inscribed circle produces a substantially short scan line (as opposed to an elliptical or circular scan spot), which is desirable in some applications such as linear detector arrays.
The present invention is designed to substantially reduce or eliminate the problems associated with wobble, such as across-scan error, and to provide a non-restrictive scanning angle. Additionally, the present invention is designed to produce a substantially elliptical or circular scan spot of significantly small size because of reduced aberrations in order to provide a very high resolution per scan.